1. Field of the Invention
The present invention generally relates to sphygmomanometers and, more particularly, to an arrangement for finely adjusting the position of a movable indicator of a pressure-indicating device of such syphgmomanometers.
2. Description of the Prior Art
A sphygmomanometer is an apparatus for measuring the blood pressure. It comprises an inflatable rubber-bag cuff which is wrapped around the upper arm of a person's arm. The cuff is connected by rubber tubing to a resilient hand bulb and is inflated by repetitively squeezing the bulb. A pressure-indicator device or pressure gauge having an indicator needle is connected by rubber tubing to the cuff, and a manually-operable valve is provided to slowly bleed air from the inflated cuff.
In use, sufficient pressurized air is pumped into the rubber cuff to compress the brachial artery in the upper arm. A stethoscope is applied over the artery below the cuff, and air is gradually allowed to escape through the valve from the cuff until the pulse can be heard. The indicator needle of the gauge at this point indicates the systolic pressure or the highest pressure in the arteries during contraction of the heart. As deflation of the cuff continues, the needle successively indicates lower and still lower pressure readings. The diastolic pressure, or lowest pressure in the artery during diastole, or relaxation of the heart muscle between beats, is indicated by the needle on the scale when the last sound of the disappearing pulse is heard. Upon further deflation of the cuff, the needle returns from its two previous measurement positions to its starting position. The normal systolic reading of an adult varies from 110 to 130 or 140 mm. of mercury. Normal diastolic readings vary from 60 to 90 mm. of mercury.
The indicator needle of the basic aneroid sphygmomanometer now in common use is operatively connected to a rotatable sector gear. The sector gear is connected to an adjustment plate which in term is pivotally connected to one end of a movable actuating rod. The other end of the rod is in force-transmitting relationship with a bellows. The interior of the bellows communicates with the interior of the cuff, and therefore as the cuff inflates and deflates, the bellows respectively expands and contracts and the actuating rod moves in opposite directions to thereby effect rotation of the sector gear with concomitant movement of the indicator needle.
For measurement accuracy, it is desirable to set the initial starting position of a floating needle to a predetermined position, e.g. a null scale reading, prior to performing any pressure measurements. However, even after the needle has once been properly set to the predetermined starting position, the needle tends not to return accurately to this predetermined starting position after continued usage due to, inter alia, temperature changes, metal expansion and contraction, changes in the spring constant of the needle return spring, shock, vibration, changes in atmospheric conditions, non-uniform expansion and contraction of the bellows, etc.
It has been proposed in the prior art to coarsely adjust the position of the indicator needle by shifting the adjustment plate relative to the sector gear. For example, in one prior art proposal, the adjustment plate is provided with an elongated slot, and a locking screw extends through the slot to the sector gear. The adjustment plate is manually shifted so that the locking screw rides in the slot until the correct spacing exists between the adjustment plate and the sector gear. The locking screw is now located in a selected location along the length of the slot and, upon tightening of the screw, the adjustment plate is fixed in position relative to the sector gear.
In another proposal of the prior art, the adjustment plate is again provided with an elongated slot, and a locking screw is again mounted for movement in this slot. In contradistinction to the first example described above, the actuating rod is not connected directly to the adjustment plate, but instead, the actuating rod is pivotally connected to an adjustment shaft which is slidably mounted in a holder which is mounted on the adjustment plate. The adjustment shaft is locked in position by a set screw mounted on the holder. In order to adjust the position of the floating needle, a technician must first loosen the set screw and manually shift the adjustment shaft. If this coarse adjustment is not sufficient to obtain the desired spacing between the adjustment plate and the sector gear, then the locking screw is loosened and the adjustment plate is thereupon also manually shifted.
However, such known prior art adjustment arrangements are disadvantageous because inadequate control is exerted over the shifting of the adjustment plate relative to the sector gear. The manual shifting of the adjustment plate and/or the manual shifting of the adjustment shaft are at best coarse adjustments. Shifting these parts to accurately position the needle, either in the manufacture or subsequently in the field, requires a high degree of skill for a technician. It frequently occurs that a technician will manually overshift the plate and/or shaft, thereby requiring another manual shifting in order to correct the first overshift, and vice versa. The frequent repetitive back and forth shifting of the various parts consumes a great deal of time. I have found it to be customary that even a skilled technician takes about 20 minutes on the average to properly adjust one pressure gauge. The coarse adjustment devices of the prior art are therefore expensive to manufacture particularly in mass production, require trained personnel, and are very costly to maintain in the field.